This invention is related to the fields of polymerization and near-net-shape production of precision objects. More specifically, it is related to the formation of articles having a surface composition distinct from that of the interior core material. Furthermore, it is related to the production of such objects where the surface and the core are an integral, monolithic entity and to the articles so produced.
Numerous applications exist that require the formation of polymeric objects with a surface composition distinct from that of the core material. For example, ophthalmic lenses can be tinted to create sunglasses or photochromic lenses. Tinting involves the absorption of dye into the surface layer of the lens. The current practice is to first create a clear lens, often by grinding and polishing a lens blank into the precise shape/contour. In certain instances, injection molding can be employed to create the prescription. The finished lens is then dipped in a dye-containing solution at elevated temperatures (e.g., in near-boiling-temperature aqueous or organic solutions). High temperatures are needed to soften and dilate the lens material to allow penetration of the dye molecules into the tight plastic network constituting the lens. This tinting (dye absorption or uptake) process is slow, even under such severe conditions. The use of high temperatures can cause dye degradation (thus necessitating frequent bath replenishment) and often leads to lens warpage. Photochromic dyes are known for their tendency toward thermal degradation, making photochromic lens manufacture a difficult task. Insufficient uptake of the photochromic dye often results and this is a primary reason such lenses often do not turn sufficiently dark when exposed to sunlight. Also, since dye molecules are quite large, the crosslinks in the plastic network must be fairly loose to allow for the penetration of the dye. Additionally, the choice of dye is greatly limited by the fact that the process requires water-soluble dyes that will also be dispersible in an organic resin matrix.
Certain objects, such as eyeglasses, also often require scratch-resistant surface coatings. Presently, finished lenses can be coated with a scratch-resistant material in a dipping tank, and the scratch-resistant material is then cured. Alternatively, spin coating and spray coating can be used as deposition means. Regardless of the method of application, the scratch- or abrasion-resistant coating forms a separate layer, distinct from the existing lens. Physical interactions are relied upon to ensure (often imperfect) adhesion between the coating and the lens core, and delamination of the coating often occurs. Thus, there is a need to prepare xe2x80x9ccoatedxe2x80x9d lenses where the coating and the lens core actually form a continuous, monolithic, integrated structure. Delamination of the xe2x80x9ccoatingxe2x80x9d will therefore no longer be an issue for such lenses.
Contact lens technologies have also evolved significantly since the introduction of the lenses. Small, pre-cured, xe2x80x9cbuttonsxe2x80x9d were ground and polished to create the needed prescription. Alternatively, polymer precursors can be used to fill mold cavities, which are then cured to form the finished lenses. Here, similar to ophthalmic lenses, shrinkage accompanying cure must be accounted for in the mold design. In either case, the finished lenses can subsequently be tinted by straightforward uptake of a dye, with the accompanying problems discussed above with respect to ophthalmic lenses, or by xe2x80x9cprintingxe2x80x9d a pattern using a variety of techniques. The printing process results in lenses mimicking the pigment distribution of a human iris. Tinting or printing (transfer printing, ink jet pattern deposition, or screen-printing) are all dictated by the dye uptake rate and strength of dye adhesion to the lens material.
In contrast to ophthalmic lenses, contact lenses must possess two additional properties. One is high oxygen permeability. The second is biocompatibility. Oxygen permeability has been found to be relatively high in soft, rubbery materials such as silicones. Silicones tend to exhibit oxygen affinity and rapid transport. Permeability is a product of diffusivity and solubility at steady state. Oxygen molecules in soft materials tend to exhibit at least high diffusivity if not high solubility as well. Block co-polymers of controlled morphology have been used to achieve high flux and dimensional stability. Morphology control is required to ensure optical transparency. Highly crosslinked silicones can also promote dimensional stability, which is necessary for precision of prescription. However, most polymers with high oxygen permeabilities do not exhibit optimal tissue biocompatibility. A certain degree of hydrophilicity is needed to give a xe2x80x9chydrogel-likexe2x80x9d surface layer to ensure comfort for the lens wearer. Surface modification schemes, such as oxidation and plasma treatment have been employed to achieve some level of weftability. Such processes, however, add cost to the manufacture. Creation of a surface layer also implies possible adhesion issues of the layer to the core lens. It would be preferable to have a monolithic object with surface composition differing from the core in a controlled manner.
The present invention discloses a novel approach that overcomes the above-described intrinsic drawbacks of commercially established processes and polymeric articles. It is unique in that it is an extremely economic process suitable for mass manufacture. The present invention also discloses parts, objects, and articles produced by this method. More particularly, this invention is directed to a process for the rapid in-situ near-net-shape polymerization of semi-solid-like materials to provide macromolecular networks and articles of manufacture. The articles of the invention have a surface and an interior core, the composition of the surface material being distinct from the composition of the core material while at the same time the surface and the core are an integral, monolithic entity. In addition, the articles are dimensionally stable and precise, with very little shrinkage during the cure process. Further, the present invention discloses a new class of polymerizable materials that exhibit a semi-solid-like behavior prior to cure, an affinity for bonding to various surface compositions, low inherent shrinkage upon curing (and therefore high-fidelity replication of the mold cavity), and highly optimized engineering properties of the final object.
In one embodiment of the invention, the process includes the steps of mixing together a dead polymer, a reactive plasticizer and an initiator to give a semi-solid polymerizable core composition; optionally shaping the semi-solid core composition into a desired geometry; exposing the core composition to a surface-forming or surface-modifying composition to give a semi-solid polymerizable gradient composite material (that is, a material where the core and surface compositions are different); and exposing the polymerizable gradient composite material to a source of polymerizing energy, to give a final product with a surface that is distinct from but integral with the core, and further, exhibits dimensional stability and high-fidelity replication. The article so produced can optionally be transparent and/or have resistance to impact (resilience). The resulting macromolecular network is characterized as having either i) a semi-interpenetrating polymer network (semi-IPN) of reactive plasticizer wrapped around and within an entangled dead polymer; or ii) an interpenetrating crosslinked polymer network of reactive plasticizer within an entangled dead polymer, the reactive plasticizer polymer network being further crosslinked to the dead polymer; or iii) interpenetrating reactive plasticizer polymer chains, which may be linear, branched, etc., within an entangled dead polymer.
The reactive plasticizer may react with the dead polymer chains if the polymer has crosslinkable groups. In the presence of multifunctional monomers, two polymer networks are formed that are crosslinked together. Grafting reactions by chain transfer to the dead polymers may also occur in addition to the reactive plasticizer network formation among the dead polymers. Such systems are desirable because crosslinking of the dead polymer to the network formed by the reactive plasticizer can prevent phase separation between the two polymer networks. Thus, the dead polymers become part of the finished network having a composition that depends on the position within the object. That is, the local composition within the finished part varies from the surface to the core.
In another embodiment of the present invention, the process includes the steps of mixing together at least one reactive plasticizer, which is preferably highly viscous, and an initiator to give a semi-solid polymerizable core composition; optionally shaping the semi-solid polymerizable core composition into a desired geometry; exposing the core composition to a surface-forming or surface-modifying composition to give a semi-solid polymerizable gradient composite material (that is, a material where the core and surface compositions are different); and exposing the polymerizable gradient composite material to a source of polymerizing energy, to give a final product with a surface that is distinct from but integral with the core, and further, exhibits dimensional stability and precision. The resulting macromolecular network is characterized as a crosslinked network of reactive plasticizer.
The final product can optionally be transparent and/or resilient.
This invention permits a broad selection of reaction chemistry to achieve precision parts with the required mechanical, thermal, optical and other desired properties. Such articles exhibit superior core characteristics (such as mechanical, diffusivity, or permeability), while having desirable surface properties (such as color, tissue biocompatibility, or barrier property). It obtains precision parts that are stress-free and flawless, with little or no birefringence. Precision products can be manufactured that are very impact-resistant or that have a high oxygen permeability, low density, or other desirable but previously difficult-to-achieve characteristics.